1. Technical Field
The present invention relates to an antireflection film and an optical element including the antireflection film.
2. Description of Related Art
An antireflection film is provided on a surface of a lens or a cover glass used in a photographic camera (including a digital camera), a broadcast camera, a monitoring camera installed on a vehicle or outdoor, and an astronomical telescope, etc., for suppressing a loss of a transmitted light, or preventing a ghost or flare. The antireflection film having a following film structure is proposed: the antireflection film is designed so that a low refractive index film and a high refractive index film are laminated, and a reflected light generated on an interface between each layer, and a light beam incident on each layer are canceled each other by interference.
For example, patent document 1 discloses an antireflection film composed of four layers having a reflectance of 0.5% when an incident angle of the light beam is 0 degree or more and 25 degrees or less, and a reflectance of 3.5% or less when the incident angle of a light beam is 0 degree or more and 60 degrees or less, when the light beam with a wavelength region of 400 nm or more and 700 nm or less is incident on an optical surface of an optical member. Further, patent document 2 discloses an antireflection film composed of 12 layers having a reflectance of less than 1% measured at an incident angle of 5 degrees in a wavelength range of 280 to 700 nm. In addition, patent document 3 discloses an antireflection film composed of 11 layer-laminated film and an antireflection film composed of 13 layer-laminated film.
The ghost and the flare are generated as follows: alight incident on a lens is reflected by a lens surface, and a light reflected thereby is incident on an imaging surface. Therefore, in order to reduce the ghost and the flare, it is effective to provide the antireflection film on the lens surface, having a low reflectance in a visible light region which can be perceived by a human eye.    Patent Document 1:    Japanese Patent Laid Open Publication No. 2005-284040    Patent Document 2:    Japanese Patent Laid Open Publication No. 2008-26820    Patent Document 3:    Japanese Patent Laid Open Publication No. 2010-217445
In recent years, a higher optical performance has been requested in an optical system, and therefore improvement of a performance of the antireflection film is also requested accordingly. Also, in recent years, a larger diameter and a wider angle of a lens is in progress. Accordingly, an incident angle range of the light beam incident on the lens is likely to be expanded. When the incident angle range is expanded, the ghost is easily generated.
In a case of a concave meniscus lens with an optical surface formed into a spherical surface, a curvature of the optical surface is fixed, and therefore a ghost light reflected by the optical surface is likely to be uniformly dispersed on an imaging surface. Meanwhile, in a case of a concave meniscus lens with the optical surface formed into an aspherical surface, a variation rate of a surface angle or the curvature of the optical surface is not formed uniformly. Therefore, the ghost light reflected by the optical surface is likely to be concentrated on apart of the imaging surface. Therefore, in a spherical concave meniscus lens, a remarkable red ghost is sometimes generated in a part of an image when the antireflection film having the same spectral reflectance property as the spherical concave meniscus lens, is formed on an aspherical concave meniscus lens, even if the ghost is sufficiently suppressed to be low by forming the antireflection film on the lens.
In a related art, the reflectance in the visible light region is uniformly suppressed to be low by forming the antireflection film into a multilayer film, and by suitably setting a parameter such as a constitutional material, a refractive index, and a film thickness of each layer.
However, in recent years, it is difficult in some cases to sufficiently satisfy an optical performance requested in an optical system only by uniformly suppressing the reflectance to be low in the visible light region.
A main object of the present invention is to provide the optical element including the antireflection film capable of satisfying the optical performance requested in the optical system in recent years, and capable of effectively reducing the ghost, even in a case of the aspherical lens, particularly such as the concave meniscus lens with the optical surface formed into the aspherical surface.